This invention relates to the fabrication of semiconductor devices and, more particularly, to a method for depositing a thin passivating film on sub-micron-size semiconductor devices included on an integrated-circuit (IC) chip.
Plasma-enhanced chemical-vapor-deposition (PECVD) processes are widely employed in the fabrication of semiconductor devices. Such processes can produce amorphous dielectric films at relatively low temperatures (about 300 degrees Celsius) with close control of composition. For a detailed description of typical PECVD processes, see, for example, Silicon Processing for the VLSI Era, Vol. 1, "Processing Technology", Luttice Press, Calif. (1986).
Deposition of a conformal passivating dielectric film is a required step in the fabrication of a variety of semiconductor devices of practical importance. Thus, for example, silicon nitride passivation of III-V compound high-electron-mobility transistors (HEMTs) is typically employed to ensure stable long-term performance of such transistors.
The high-frequency operation of an HEMT device is a function of the thickness of the dielectric deposited on its gate electrode. As this thickness increases, the high-frequency performance of the transistor device is degraded due to an increase in gate capacitance.
In a conventional PECVD process carried out at about 300 degrees Celsius, the thickness of the dielectric on the gate electrode is typically not determined solely by the thickness of the film deposited during the gate passivation step. The final dielectric thickness thereon is also determined by the additional thickness of dielectric material deposited in other PECVD steps of the fabrication sequence. Thus, for example, when dielectric material is deposited to form capacitors on the IC chip, this additional material adds to the thickness of the passivating film to produce a dielectric on the gate electrode that is thicker than that desired for optimal high-frequency performance.
In an HEMT device designed to operate at very high frequencies, the gate electrode of the device may have a width in the sub-micron range [for example, in the range of only about 0.1 micrometers (.mu.m) to one .mu.m]. To reduce the resistivity of such a small gate electrode, and thereby preserve its high-frequency performance, the electrode is typically formed to have, for example, a mushroom-type shape. But, in practice, the task of achieving complete conformal coverage of the entirety of the surface of such an irregularly shaped gate electrode with a PECVD-deposited passivating film has been found to be extremely difficult, if not impossible. And, without such complete passivation, the uncovered portions of the gate electrode may, for example, be subsequently eroded or oxidized, thereby to deleteriously affect the performance of the final device.
Accordingly, efforts have continued by workers skilled in the art directed at trying to devise other ways of depositing a thin conformal passivating film on sub-micron-size semiconductor devices. It was recognized that such efforts, if successful, could improve the performance and reliability of devices designed to operate at very high frequencies.